1. Field of the Invention
The present invention relates to melt stabilized thermoplastic compositions, and more particularly relates to melt stabilized polyesters blend compositions.
2. Description of the Related Art
Thermoplastic blends of low molecular weight polyester resin and high molecular weight polyester resin are known, See Walsh, U.S. Pat. No. 5,367,011, issued Nov. 22, 1994, which is incorporated herein by reference; and thermoplastic blends of polyester resin and aromatic polycarbonate resin are known, see Roovers et al., U.S. Pat. No. 4,965,308, issued Oct. 23, 1990, which is incorporated herein by reference. As set out in the above references, such blend compositions can experience instability due to transesterification of the resins in the blends. Walsh, U.S. Pat. No. 5,367,011 teaches utilizing phosphorus containing compounds in blends of high molecular weight polyester resin and low molecular weight resins; and Roovers et al., U.S. Pat. No. 4,965,308, teaches stabilizing mixtures of aromatic polycarbonate and aromatic polyester with a sulphurous compound such as zinc sulphate (ZnSO.sub.4 .cndot.7HOH), sulphuric acid, sulphurous acid, metal sulphites, metal bisulphites, metal hydrogen sulphate and metal thiosulphate.
Additionally, various sulphurous compounds such as alkyl-aryl sulphonates have been disclosed as antistatic additives for polyesters, and sodium lauryl sulphate has been set out in EP8911384 as an unsatisfactory additive for polyesters due to being hard to mix with polyester in an extruder. Sulphonate-type anionic antistatic agents have been disclosed as additives for polybutylene terephthalate resin, see EP0185555 published Jun. 25, 1986. However, additives such as alkyl-aryl sulfonic acid sodium salts are generally not effective in preventing molecular weight build up in polyester blends or in suppressing polycarbonate-polyester interreactions. Additionally, various additives can have undesirable drawbacks or lack effectiveness in stabilizing the polyester blend compositions.
Consequently, there is a desire to provide effective melt stabilizers for polyester blend compositions, and preferably to provide stabilizers which are readily available, inexpensive and safe for use in such blends.